Garbage collection (by alaric)
The simplest (and, perhaps, most widespread) technique is reference counting. In this scheme, at every place in the program where something keeps a reference to a memory block for later use, an instruction is added to increase a "reference count" stored in the referenced object. And at every point where a reference is no longer needed, an instruction is added that reduces the reference count in the object - and if the reference count is now zero, frees up the memory block (after first finding all the objects it itself refers to, and reducing their reference count).
This technique is nice and simple to understand, but has a few issues:
- All this updating of reference counts is a bit slow, and has to happen at critical points during the running of the program.
- It is prone to programmer error, although not as much as manual management - the programmer could miss out a reference count increase, meaning that a bit of memory is freed before it's finished with leading to a crash; or they could miss a reference count decrease, leading to a bit of memory never reaching a count of 0, and thus never being freed. It's better than manual management in that a programmer does not need to vouch for whether or not the whole program has finished with a bit of memory; just the little module he or she is writing at that point. If other bits of the program are still using that memory, this will be reflected in the reference count.
- It has a problem with loops. If two memory blocks each contain a reference to the other, then even if they are no longer being used, they will never be freed - since they keep their references to each other, each will have a reference count of 1.
Some programming languages automate the reference counting, removing the second disadvantage; when you assign to a reference, if the old value is non-null, they decrement the reference count of the referenced object; then if the new reference is non-null, they increment the reference count of the referenced object. This removes the risk of programmer error, but it's still a bit slow, and has the loop problem.
So the search went on for better techniques.
Wed 16th Nov 2005 11:29 pm 4 Comments
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By Faré, Fri 18th Nov 2005 @ 3:21 am
So as to be able to move your object to the top of the list, yet preserve the invariant that no object points to a newer object, you need to also move up all the objects that point to your born-again object, and so on recursively. In practice, this is only affordable if your object has no one pointing to it except the current context -- in which case what you have is a linearity constraint on objects.
By Alaric Snell-Pym, Fri 18th Nov 2005 @ 10:31 am
Ah, but I do have a linearity constraint on objects 😉 That's why I was amused by the similarity to "copy on write" handling of tree structures on disk, which basically works the same way (to safely update a tree structure like a B-Tree, make copies of the changed leaf nodes into empty space, then work out what intermediate nodes would need changing to reflect the new locations of the leaf nodes and do the same with them, then continue bubbling the change up the tree until you have a new root pointer, etc).
I'm sure there's some Fundamental Truth in the fact that the same underlying technique turns out to be useful both on disk and in memory, yet in very different contexts (ACID properties on disk, fast garbage collection in RAM).
By Alaric Snell-Pym, Sun 20th Nov 2005 @ 2:33 pm
Oooh, while sawing up logs (a great time for thinking about abstract stuff) I was struck by a flaw...
When a memory block is modified and gets brought up to the head of the chain, IF the collector has not yet reached the block in this pass, then the blocks referenced by this block will not get marked since the collector will then not visit the block in question until the next pass. If nothing else refers to the same blocks, they'll be freed. Oops!
So we need to make sure that a block moved to the top of the chain still gets seen. My first thought was that the application could just quickly scan the block and mark all the referenced blocks, but that's wrong - it's the collector's job.
So my next thought was to have a (either shared and lock-free, or per-processor, to stop it from becoming a point of contention in SMP systems) stack of 'touched' objects; when altering an object, the application would merely need to push a reference onto this stack (it wouldn't even need to do the move to the head of the chain). Now, the collector, whenever it's about to examine the next object in the chain, would first look on the stack(s) and go through any memory blocks on them, marking all the referenced blocks. That way, it will never be considering a block for freeing unless it has already 'scanned' all modified objects, so there's no chance of it mistakenly freeing something. Whenever the collector has scanned a block from the stack, it can then also do the chore of moving that block to the head of the chain, moving the task from the application code.
However, there is a problem - an application that just sits there modifying the same large array of pointers over and over again would keep the collector forever rescanning that large array; never getting any real collection done. What we need is to only stack memory blocks for scanning if they've not yet been scanned anyway. This is easily resolved; have a 'scanned' flag in each block, that the collector sets whenever it scans a block, be it due to the block being on the stack or by traversing the chain. Newly allocated objects also have the 'scanned' flag set, since all the objectss they refer to must have been reachable anyway, and thus will be marked - they don't need rescanning until the next pass. When the collector finishes scanning the chain and is about to start again, it has to clear all the 'scanned' flags; but rather than walking the chain doing this, it's easier to just reverse the interpretation of the 'scanned' flag. Then newly created blocks will need to be marked as 'unscanned' for the next scan.
There's a potential race condition in that if the collector changes the global variable that says what newly created blocks should be marked as between the application reading the current setting and the application putting the newly created block at the head of the chain, it could end up with the wrong setting. Therefore, before doing the swap, the collector should take a copy of the pointer to the current head of the chain; then when it starts its walk of the chain, it should force the correct value into all the blocks it examines until it hits the point in the chain it marked, this way ensuring nothing gets missed.